With the development of international communication services, especially the rapid development of internet technology, 3G technology, and passive optical network technology, the demand of the communication systems on the bandwidth of optical fiber is becoming increasingly high. In the communication systems of long distance, large capacity, and high speed transmission, the optical fiber amplifier technology and wavelength division multiplexing technology are generally used. Especially in backbone networks and undersea communication networks, the requirement for the transmission distance without relay and transmission capacity of the optical fiber is much higher. However, with the increasing of transmission capacity and transmission distance, a higher input power and a lower attenuation of the optical fiber are needed to meet the requirement of distinguishable signal-to-noise ratio. With the increasing of the input power of the optical fiber, the self-phase modulation, cross-phase modulation, four-wave mixing, and other nonlinear effects, especially the stimulated Brillouin scattering effect which has a relatively low threshold, would occur inevitably in the narrow core layer of the optical fiber. The occurrence of these effects would result in crosstalk phenomenon in the system, or reduce the signal-to-noise ratio of the system. In this case, the transmission capacity of the communication system cannot be improved any further.
The occurrence of these nonlinear effects is particularly related to the optical power density of the optical fiber. In general, if the effective area of the optical fiber is improved, the power density of the optical fiber can be reduced. In this manner, the thresholds of the nonlinear effects of the optical fiber can be reduced, and the transmission power thereof can be improved accordingly. However, with the increasing of the effective area, the value of MAC of the optical fiber will increase accordingly, and the optical fiber will become more sensitive to the bending thereof. In practical use, the additional attenuation generated by the bending of the optical fiber would increase the total attenuation of the optical fiber, and thus the transmission performance thereof would be affected. In addition, if the attenuation of the optical fiber can be reduced, the signal with the same input power can be transmitted farther in the optical fiber, and thus the transmission capacity thereof can be improved.
In the G.654 standard revised in 2010 by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), a cutoff wavelength shifted single-mode optical fiber is defined. The attenuation value of this kind of optical fiber is less than 0.22 dB/km, and the mode field diameter thereof at a wavelength of 1550 nm ranges from 9.5 μm to 13 μm. Compared with Standard Single-Mode Fiber (SSMF), the mode field diameter of the above cutoff wavelength shifted single-mode optical fiber is 1 μm to 2 μm higher. Therefore, the aforesaid cutoff wavelength shifted single-mode optical fiber has a relatively large effective area. When a cable made of said optical fiber is used undersea, the span distance without relay thereof can be improved effectively. However, its macro-bending performance is inferior to SSMF apparently.
An optical fiber is proposed by U.S. Pat. No. 6,904,218. Said optical fiber comprises a central core, an intermediate depressed cladding layer, and an outer cladding layer. The effective area of the optical fiber is larger than 80 μm2 at a wavelength of 1310 nm; the macro-bending attenuation thereof with a bending radius of 10 mm is less than 0.7 dB/turn; and the attenuation value thereof is less than 0.19 dB/km. However, in all of the embodiments of U.S. Pat. No. 6,904,218, the effective area of the optical fiber is 131.2 μm2 at most at a wavelength of 1550 nm.
Another kind of optical fiber is proposed by U.S. Pat. No. 7,254,305. Said optical fiber comprises a central core, an intermediate cladding layer, a depressed cladding layer, and an outer cladding layer. The attenuation value of the optical fiber is less than 0.19 dB/km at a wavelength of 1550 nm. However, the refractive index of the central core is far different from that of the cladding layer, which renders that an effective area larger than 100 μm2 cannot be obtained. Meanwhile, since the absolute value of the refractive index of the central core is relatively high, a high germanium-doping concentration is needed, and thus the attenuation value of the optical fiber is higher than 0.185 dB/km.
In general, the effective area of the optical fiber can be improved through the following two methods. On the one hand, the effective area of the optical fiber can be improved through increasing the geometric dimension of the core layer. The refractive index of the core layer is higher than that of the cladding layer, and most of light transmits in the core layer of the optical fiber. Hence, the effective area of the optical fiber can be improved directly through increasing the diameter of the core layer. However, the increasing of the diameter of the core layer will affect the cutoff wavelength of the optical fiber directly, while the cutoff wavelength must be less than the window wavelength of the communication. Therefore, the increasing extent of the core layer diameter is limited. On the other hand, the effective area of the optical fiber can be improved through reducing the relative refractive index of the core layer. In this manner, the optical field can be distributed flatter, the effective area can be increased, and the cutoff wavelength can be reduced. However, in this case, the attenuation of the optical fiber would be adversely affected.
Although the effective area of the optical fiber can be improved through changing the structure of the core layer and increasing the size of the core layer, the bending performance and the attenuation performance of the optical fiber would be adversely affected. Taking the bending performance of the optical fiber into consideration, the increasing of the effective area of the optical fiber in the above patents is limited. At present, an optical fiber with an effective area larger than 135 μm2 and good bending performance is not reported yet by any patent.